Fix Javadoc for ObservationThreadLocalAccessor(ObservationRegistry)

[izeye]

This PR sets the instance static field in the ObservationThreadLocalAccessor(ObservationRegistry) constructor as it seems to have been missed based on its Javadoc.

[marcingrzejszczak]

I don't think we should be setting this because for the ContextRegistry default instantiation, the default constructor will be called. If someone is manually updating a ContextRegistry then they should manually update the instance. They might also be using an instance for tests and don't want the test instance to be used in other parts of the system.

[izeye]

@marcingrzejszczak Thanks for the feedback! I fixed its Javadoc instead.

[marcingrzejszczak]

Thank you!